Polypropylene foam sheets

ABSTRACT

A thermoformable, rigid or semi-rigid polypropylene foam sheet having a smooth surface and a uniform cell structure and a density of at least 2.5 lbs/ft 3  is prepared by extruding a mixture of a nucleating agent, a physical blowing agent and a polypropylene resin having a high melt strength and high melt elasticity.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to polypropylene foam sheets and a process fortheir manufacture. Specifically, this invention relates to polypropylenefoam sheets which are rigid or semi-rigid and thermoformable into shapedarticles for use in packaging and service applications.

2. Description of the Prior Art

A foamed plastic or cellular plastic has an apparent density which isdecreased by the presence of numerous voids or cells dispersedthroughout its mass (ASTM D883-80C). The cells may be interconnected(open-celled) and/or discrete and independent (closed-celled).

The prior art discloses various methods for the preparation of foamedplastics. These include leaching out a solid or liquid which isdispersed in a plastic, sintering small particles of a plastic anddispersing cellular particles in a plastic. However, the most widelyused method involves the dispersion of a gaseous phase throughout afluid polymer phase and the retention of the resultant expanded form.

The theory of the expansion process and the properties of various foamedplastics are reviewed in "Cellular Plastics", in Encyclopedia of PolymerScience and Engineering, vol. 3, pp. 1-59 (1985), which is incorporatedherein by reference. As disclosed therein, the expansion processconsists of three steps: creation of small discontinuities or cells in afluid or plastic phase, growth of these cells to a desired volume andstabilization of the resultant cellular structure by physical orchemical means.

The formation of discontinuities or bubbles within the fluid polymer,may arise from gases that are injected into the fluid polymer, lowboiling liquids that are incorporated into the system as blowing agentsand volatilize due to increased temperature or decreased pressure, gasesthat are produced as a result of a chemical reaction within the fluidpolymer and chemical blowing agents which undergo thermal decompositionto form a gas.

The rate of growth of the bubbles or cells depends upon the viscoelasticnature of the polymer phase, the blowing agent pressure, the externalpressure on the foam, the cell size and the permeation rate of theblowing agent through the polymer phase.

Cell or bubble stabilization relates to cell wall stability and thedrainage of material from the membrane or wall which separates cells.Increasing the viscosity of the fluid reduces the drainage effect. Theviscosity increase may be caused by chemical reactions which increasemolecular weight through polymerization or crosslinking, or bytemperature reduction, ultimately below the second order transition orcrystallization temperature to prevent polymer flow.

The present invention relates to rigid or semi-rigid foam sheets for usein food service applications. The prior art has utilized polystyrene forthe manufacture of foam sheets for these applications. However,polystyrene articles suffer from low service temperature, and little orno photochemical or biological degradability and are relativelyexpensive.

Polypropylene does not have these undesirable characteristics. Variousprocesses have been reported in the prior art for the preparation offlexible or rigid polypropylene foams. The processes are designed topromote the three-step process described hereinbefore, i.e. creation ofcells in a fluid or plastic phase, growth of the cells and stabilizationof the resultant cellular structure.

Blowing agents used in the preparation of polypropylene foam includeazodicarbonamide (Lee et al, J. Appl. Polym. Sci. 32, 4639 (1986); EPOPat. Appl. EP 190,021), chlorofluorocarbons (EPO Pat. Appl. EP 1791, EP71,981, EP 181,637; U.K. Pat. 1,400,494; U.K. Pat. Appl. GB 2,099,434A), carbon dioxide (EPO Pat. Appl. EP 291,764), hydrocarbons, e.g.propane, butane, pentane (U.K. Pat. 1,400,494; U.K. Pat. Appl. GB2,099,434 A) and water (EPO Pat. Appl. EP 122,460).

Crystallization rate accelerators and/or nucleating agents used in thepreparation of polypropylene foam include titanium dioxide (EPO Pat.Appl. EP 122,460; U.K. Pat. Appl. GB 2,099,434 A talc (U.K. Pat.1,400,494; U.K. Pat. Appl. GB 2,099,434 A), silica and silicates (EPOPat. Appl. EP 1791; U.S. Pat. 4,467,052), zeolite 4A (EPO Pat. Appl. EP178,282, EP 178,283), sodium benzoate (Colton, Plast. Eng. 44(8), 53(1988) and dibenzylidene sorbitol (EPO Pat. Appl. EP 178,282).

Citric acid-sodium bicarbonate combinations are considered as blowingagents in some patents and as nucleating agents in other patents (EPOPat. Appl. EP 178,283; U.K. Pat. 1,400,494; U.K Pat. Appl. GB 2,099,434A; U.S. Pat. 4,467,052).

The use of crosslinking agents during the preparation of a polypropylenefoam has been reported in the prior art and include peroxides (Nojiri etal, Furukawa Review 2, 34 (1982) through Chem. Abstracts 97, 21725ou(1982); EPO Pat. Appl. EP 181,637, 190,021) in the absence or presenceof multifunctional vinyl monomers, azido functional silanes (EPO Pat.Appl. EP 181,637), vinyltrimethoxysilane (Lee et al, J. Appl. Polym.Sci. 32, 4639 (1986) and ionizing radiation in the presence ofpolyacrylic monomers (Nojiri et al, Furukawa Review 2, 34 (1982); U.S.Pat. No. 4,424,293).

Low density polypropylene foams "free from creases on the surface" havebeen prepared by incorporating high molecular weight fatty amides,amines or esters in the molten polyolefin (EPO Pat. Appl. EP 1791).

The prior art teaches that polypropylene is not a unique material, i.e.processes that are applicable to the preparation of foam ormicrocellular structures from other polymers are applicable to thepreparation of polypropylene foams.

EPO Pat. Appl. EP 1791 describes "a process for the preparation ofexpanded thermoplastic synthetic resins" and discloses polyethylene,ethylene-vinyl acetate copolymer and isotactic polypropylene as theapplicable thermoplastic resins.

EPO Pat. Appl. EP 71,981 describes "foamed polypropylene resin moldedarticles" and discloses the use of ethylene-propylene copolymer as wellas polypropylene.

EPO Pat. Appl. EP 122,460 describes "resin foam produced by an aqueousmedium" and discloses polymer foams from polypropylene, polyethylene andpolystyrene.

EPO Pat. Appl. EP 291,764 describes the "extrusion of propylene polymerfoam sheets" and discloses a process for extruding blends ofethylene-propylene block copolymers containing less than 20% ethylenewith block copolymers containing less than 5% ethylene or randomethylene-propylene copolymers or polypropylene.

U.K. Pat. 1,400,494 describes "foamed polymeric sheet material andprocess therefor" and discloses polypropylene, high density polyethyleneand nylon-12 as the preferred operable polymers while indicating thatcopolymers of ethylene with vinyl acetate or vinyl chloride can beconveniently used.

U.K. Pat. Appl. GB 2,099,434 A describes an "extrusion process forpropylene resin foams" and states that the resin may be isotacticpolypropylene, an ethylene-propylene block or random copolymer or blendsof polypropylene with numerous olefin homopolymers and copolymers.

U.S. Pat. No. 3,637,458 describes "microcellular foam sheet" from alinear, thermoplastic crystalline polymer and claims isotacticpolypropylene and linear polyethylene foam sheet.

U.S. Pat. No. 3,819,784 describes "a process for preparing moldedpolyolefin foam" and discloses that suitable polyolefins used in theprocess include low density polyethylene, high density polyethylene,isotactic polypropylene, poly-1-butene and copolymers of ethylene withpropylene or vinyl acetate.

U.S. Pat. No. 3,830,900 describes "method of forming foamed plasticsheets" and discloses that the method is applicable to polyvinylchloride, polystyrene, polyethylene, polypropylene andacrylonitrile-butadiene-styrene copolymers.

U.S. Pat. No. 4,467,052 describes a "tray for packaging food products"and discloses an injection molding process for the preparation of foamtrays from blends of polypropylene and styrene-butadiene rubber.

Colton (Plast. Eng. 44(8), 53 (1988) describes "making micro cellularfoams from crystalline polymers" and discloses microcellularpolypropylene and ethylene-propylene copolymer foams.

EPO Pat. Appl. 181,637 describes "lightly crosslinked linear olefinicpolymer foams" prepared from melt blends of one or more polymersselected from high density polyethylene, linear low densitypolyethylene, polypropylene and polystyrene.

EPO Pat. Appl. EP 190,021 describes "heat-foamable cross-linkedpropylene resin compositions" and discloses blends of propylene-α-olefincopolymers or 1-butene-α-olefin copolymers with polypropylene.

U.S. Pat. No. 4,424,293 describes "crosslinkable polypropylenecomposition" and discloses foams from isotactic polypropylene andethylene-propylene copolymer.

The prior art uses "polypropylene" as a self-explanatory term for apolymer prepared from propylene monomer. In some cases the terms"isotactic polypropylene" and "crystalline polypropylene" are used. Inonly a few patents is the polypropylene characterized to any furtherextent.

EPO Pat. Appl. EP 71,981 discloses polypropylene foams prepared fromresins having a latent heat of crystallization of 9-28 cal/g. U.S. Pat.No. 3,637,458 discloses polypropylene foams prepared from polymers of"at least film forming molecular weight, substantially free fromcrosslinking, and having a work-to-break (WTB) value of at least 10,000inch-lbs/inch³ ". U.K. Pat. Appl. GB 2,099,434 A discloses polypropylenefoams prepared from resins having a melt tension of at least 3 grams at190° C. and a maximum/minimum melt tension ratio of not more than 2.5/1.

Application of the processes of the prior art to generic or commercialpolypropylene resins, described as polypropylene, isotacticpolypropylene or crystalline polypropylene, fails to yield thepolypropylene foam sheet of the present invention.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a foam sheet materialhaving a high impact strength, a modulus suitable for rigid orsemi-rigid packaging applications and convertible into trays, plates,containers and other articles used in food service.

Another object of the instant invention is to provide a thermoformablefoam sheet which has a high service temperature.

A further object of the present invention is to provide a foam sheethaving high insulation properties and cost advantages over existing foamsheets.

Yet another object of the present invention is to provide a foam sheetprepared from a polypropylene resin and having a density ranging from2.5 to 25 lbs/ft³ and a modulus of at least 10,000 psi.

Still another object of the present invention is to provide apolypropylene foam sheet having a uniform cell structure and smoothsurfaces.

It has now been found that these improvements in a foam sheet can beachieved by extruding high melt strength, high melt elasticitypolypropylene, characterized by at least (a) either high M_(z) or highM_(z) /M_(w) ratio, and (b) either high equilibrium compliance J_(eo)obtained from creep measurements or high recoverable strain per unitstress Sr/S obtained from steady shear measurements.

In one embodiment, the present invention provides a rigid or semi-rigidpolypropylene foam sheet having a density ranging from 2.5 to 25lbs/ft³, tensile and flexural moduli of at least 10,000 psi, a cell sizeof 5-18 mils and a thickness ranging from about 0.02 to 0.20 inches. Thepolypropylene foam sheet is thermoformable and has uniform cellstructure and smooth surfaces.

In another embodiment of this invention, a process is provided forproducing the polypropylene foam sheet of the invention. The process maybe conducted using a single or tandem extrusion line. The latter ispreferred and, by the use of primary and secondary extruders in series,a continuous foam sheet is produced. The process comprises mixingpolypropylene resin, having a high melt strength and a high meltelasticity, with a nucleating agent in the primary extruder,plasticating the mixture, injecting a physical blowing agent into theplasticated mixture to form a foaming mixture, which is transferred to asecondary extruder, mixing and cooling the foaming mixture and extrudingthe foaming mixture through an annular or flat die into a continuousfoam sheet.

In another embodiment of this invention, a method is provided forforming rigid or semi-rigid articles from the polypropylene foam sheetof the invention. The process comprises heating the foam sheet to atemperature which permits deformation under vacuum or pressure,supplying the softened foam sheet to a forming mold and cooling the foamsheet to form a rigid or semi-rigid article having the shape of themold.

In another embodiment of the invention, a rigid or semi-rigid multilayerstructure is provided. The multilayer structure comprises at least onelayer of the polypropylene foam sheet of the invention and at least onelayer having functional properties, e.g. barrier properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a tandem foam extrusion line.

FIG. 2 is a scanning electron microscope (SEM) micrograph of a crosssection of a polypropylene foam sheet prepared from polypropylene resinA-6.

FIG. 3 is an SEM micrograph of a cross section of a polypropylene foamsheet prepared from polypropylene resin A-7.

FIG. 4 is an SEM micrograph of a cross section of a polypropylene foamsheet prepared from polypropylene resin A-2.

FIG. 5 is an SEM micrograph of a cross section of a polypropylene foamsheet prepared from polypropylene resin A-17.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, it has been found that athermoformable polypropylene foam sheet having high modulus impactstrength, service temperature and insulation properties, may be producedby a process which comprises the steps of mixing a polypropylene resinhaving specific molecular and rheological characteristics with anucleating agent, plasticating the mixture, introducing a physicalblowing agent into the plasticated mixture to form a foaming mixture,mixing, cooling and extruding the foaming mixture through an extruderdie into a foamed extrudate which is formed into a continuous foamedsheet.

Acceptable foam sheets produced from the operable polypropylene resinsby this process, have a density between 2.5 and 25.0 lb/ft³, suitablefor semirigid and rigid packaging and food service applications and havea substantially uniform cell structure and smooth surfaces. Foams whichare unacceptable and unsatisfactory for thermoforming into objects forthe indicated applications, have non-uniform cell structures, roughsurfaces and densities outside of this range.

The base resin plays the major role in determining the foamability andthe properties of the foam products made therefrom. The polypropyleneresins which yield acceptable foams, particularly when processed by themethod disclosed herein, may be distinguished from the polypropyleneresins which yield unsatisfactory foams, by their molecular andrheoloqical characteristics.

The melt strength of a polymer is important in processes such as foamingwhere deformation is primarily elongational and tensile stresses arepresent. High molecular weight polypropylene resins are frequentlycharacterized as "high melt strength" (HMS) resins. However,unexpectedly, it has been found that this characterization is inadequateand that numerous high molecular weight polypropylene resins, designatedand marketed as "high melt strength" resins fail to yield acceptablefoam sheets.

The molecular weight distribution in a sample of polypropylene may bedetermined by high temperature gel permeation chromatography (GPC). TheWaters 150 CV GPC chromatograph may be used at 135° C. withtrichlorobenzene as carrier solvent and a set of Waters μ-Styragel HT,10³, 10⁴, 10⁵ and 10⁶ A columns. The solution concentration is 0.2(w/v)and the flow rate is 1 ml/min.

GPC provides information about (a) the number average molecular weightM_(n) which is the arithmetical mean value obtained by dividing the sumof the molecular weights by the number of molecules and thus isdependent simply upon the total number of molecules, (b) the weightaverage molecular weight (M_(w)) which is the second-power average ofmolecular weights and is more dependent on the number of heaviermolecules than is M_(n), and (c) the z-average molecular weight (M_(z))which is the third-power average of molecular weights.

Colligative properties are related to M_(n), bulk properties associatedwith large deformations such as viscosity and toughness are affected byM_(w) values and melt elasticity is more closely dependent on M_(z)values.

The polypropylene resins which are effective in yielding acceptablefoams by the process of the present invention, are of high molecularweight with an M_(z) value above 1.0×10⁶ and an M_(z) /M_(w) ratio above3.0. The polydispersity index M_(w) /M_(n) is of less significance sinceit does not differentiate between polypropylene resins which giveacceptable foams and those which give unsatisfactory foams. Resinshaving M_(z) and M_(z) /M_(w) values below the indicated values yieldfoam sheets which are unacceptable.

GPC chromatograms of resins which yield unacceptable foam sheets, usinga viscometer detector, show a unimodal molecular weight distribution andplots of the branching factor g' versus log molecular weight (M_(w)),where

    g'=[η]/KM.sup.a

show the absence of significant branching, i.e. the chains areessentially linear. In contrast, resins which yield acceptable foamsheets show a bimodal molecular weight distribution, wherein the majorcomponent is largely linear while the higher molecular weight minorcomponent is highly branched.

Melt flow rates of resins which may be utilized in the process of thepresent invention, range from 0.2 to 12 g/10 min, measured in a meltflow instrument at 230° C. under a load of 2.16 kg.

The importance of melt elasticity in the conversion of polypropyleneresins to acceptable foam sheets, indicated by M_(z) values, isconfirmed by rheological characterization of polymer melts in a shearfield.

The rheological characterization of the polypropylene resins wasconducted with a programmed Rheometrics Mechanical Spectrometer(RMS-800). Resin pellets were compression molded into sheets from whichsamples were stamped out with a 25 mm diameter circular die. Tests wereconducted at 210∓1° C. using 25 mm parallel plate geometry with a 1.4 mmgap. Creep data were obtained under a constant stress of 1000 dyn/sq.cmfor a period of 0-300 sec. The creep compliance J(t) is given by##EQU1## where ε=strain

τ_(o) =stress

J_(eo) =equilibrium compliance

η_(o) =zero shear viscosity

The equilibrium compliance J_(eo) is a measure of melt elasticity and isdetermined by first plotting strain against time at constant stress. Thestrain as a function of time is divided by the stress to give J(t).J_(eo) is the intercept of the J(t) against time plot.

Polypropylene resins which yield acceptable foam sheets, by the processof the present invention, have equilibrium compliance J_(eo) valuesabove 12×10⁻⁵ cm² /dyne. Resins having J_(eo) values below this valueyield unacceptable foam sheets with non-uniform cell structure anduneven surfaces.

The recoverable shear strain per unit stress Sr/S also distinguishespolypropylene resins which yield acceptable foam sheets from those whichyield unacceptable foam sheets. This quantity is a fundamental measureof melt elasticity. Using the programmed Rheometrics MechanicalSpectrometer, the polymer melt is subjected to clockwise rotationalshear strain by the driver and the resulting shear stress S and firstnormal stress N_(l) are measured by a transducer. The shear rate rangeis 0.01-10 s⁻¹, the time before measurement is 2.2 min and the time ofthe measurement is 0.3 min. Normal stress measurements are obtained ateach shear rate. The recoverable shear strain Sr is obtained from thefirst normal stress difference ##EQU2## The normalized quantity Sr/S,i.e. recoverable shear strain per unit stress is a measure of meltelasticity. Polypropylene resins which are extrudable to acceptable foamsheets have Sr/S values at 1 sec⁻¹ above 5×10⁻⁵ cm² /dyne, generallyabove 7×10⁻⁵ cm² /dyne, while unacceptable foam sheets are obtained fromresins with lower values.

In addition to the polypropylene resins which have the molecular andrheological characteristics indicated above, blends of polypropyleneresins with other polymers are also of use in the practice of thepresent invention, provided said blends have the necessary molecular andrheological characteristics. Thus, blends of the polypropylene resinswhich are useful in the preparation of the foam sheets of thisinvention, with other polymers such as linear polypropylene orpolyethylene resins may be used if such blends haved the requiredcharacteristics. Further, blends of linear polypropylene resins withbranched polyethylene or polypropylene resins may be used when suchblends have the necessary molecular and rheological characteristics. Thebranched polyolefins may be prepared by exposure of the linearpolyolefins to low level radiation, in accordance with U.S. Pat. No.4,525,257 or by other appropriate methods.

In accordance with the present invention, an extrusion process isprovided for converting the polypropylene resins having the necessarymolecular and rheological characteristics to acceptable foam sheets.Twin or single screw extruders may be used. Single extruders or,preferably, primary and secondary extruders, generally called tandemextruders, are effective in conducting the mixture of polypropyleneresin and additives through the necessary plasticating, mixing andcooling steps which are followed by extrusion to foam sheet havinguniform cell structure and smooth surfaces.

A tandem extrusion line is schematically represented in FIG. 1. The baseresin and the nucleating agent are added from separate feeders through asingle port to the unheated zone 1 of the primary extruder 7. Themixture is moved by the plasticating and mixing screw through the heatedzones 2-6. The blowing agent is added to the plasticating mixture inzone 4. The resultant "foaming mixture" is transferred to the secondaryextruder 8 through the heated crossover 9. Mixing and cooling occur asthe screw carries the mixture through heated zones 10-13. The melt pump14 moves the "foaming mixture" into the heated die 15 and through zones16-18. The die may be either a circular (annular) or flat die. Theextruded foaming mixture forms a foamed extrudate which is sheet-likewhen coming through a flat die, or tubular when coming through anannular die. In either case, the extrudate is cooled by an air ring 19attached to the die lip. If an annular die is used, the extruded foamtube is pulled over a cooling/sizing drum 20 where it is further cooledby an air ring 21. The extruded foam tube or sleeve is split whilepassing over the drum 20. The extrudate from either a flat die or anannular die, the latter after splitting and spreading, is then flattenedinto a foam sheet by passage over a series of rolls, e.g. an S-wrap, andthen taken up on a winder. The continuous foam sheet is then aged for aperiod of time to allow for diffusion of the blowing agent and airthrough the cell walls to bring it to equilibrium, prior to furtherfabrication, if any.

The extrusion process disclosed above is representative and not limitingas to equipment and procedural details. Alternative equipment andvariations in the procedure will be obvious to those skilled in the art.

A polypropylene foam sheet may be provided with a substantiallynon-cellular outer layer or skin. Such a skin may give the foam asuperior outer appearance in that the foam structure with skin may havea shiny or glossy appearance, and is also resistant to surface abrasionand cutting. The skin also acts as a stiffener to enable a lighterand/or thinner structure having a maximum bending stiffness to beobtained. A skin may be formed on a single layer structure by changingthe flow rate and/or the temperature of the air which is applied to thesurface of the tubular or flat extrudate coming out of a die.Alternatively, a skin layer may be formed by using a multimanifold dieor combining feedblock to coextrude a non-foamed polypropylene or otherlayer on the outside of a polypropylene foam layer.

In accordance with another embodiment of the present invention, amultilayer foam sheet is provided. This foam sheet comprises at leastone layer of the polypropylene foam sheet of this invention and at leastone functional layer. The presence of functional layers in themultilayered foam sheet of the invention can effectively act as a watervapor or gas barrier. The use of functional layers in combination withpolypropylene foam layers thus can enable a product to be produced thatis effective for use as a container which not only has the advantageousproperties of the polypropylene foam, but in addition can act as abarrier to air or water vapor and thus can be useful in packagingapplications where an extended shelf-life is desirable.

The functional layers which can be utilized in accordance with thepresent invention include ethylene-vinyl alcohol and vinylidene chloridecopolymers and polyamides. A typical multilayered foam sheetconfiguration, in accordance with the invention, might include one ormore functional layers sandwiched between two layers of polypropylenefoam sheet. Typically, the thickness of the functional layer or layerswill constitute less than about 5% of the total thickness of themultilayer construction. In cases where the materials utilized as thefunctional layer are not compatible with or adherent to thepolypropylene foam layers, it may be desirable, in accordance with theinvention, to utilize "tie" layers between the functional andpolypropylene foam layers. These tie layers may function to hold thefunctional and polypropylene foam layers together and thus act asadhesives.

Typical tie layers are based on olefin copolymers containing polarfunctionality, e.g. ester, carboxyl and amide groups, generally preparedby copolymerization of an olefin monomer or graft copolymerization of anolefin polymer with one or more monomers containing the polarfunctionality. Thus, polypropylene-maleic anhydride andpolypropylene-acrylic acid graft copolymers and the like are effectivetie layers.

In accordance with the invention, a process is provided for producing amultilayered polypropylene foam sheet. The process comprises the stepsof mixing pelletized polypropylene resin with a nucleating agent,plasticating the mixture, introducing a physical blowing agent into thesubstantially plasticated mixture to form a foaming mixture, mixing andcooling the foaming mixture, supplying the foaming mixture and aseparately plasticated functional resin to a combining feedblock ormultimanifold die of an extruder, and co-extruding the foaming mixtureand plasticated functional resin into a continuous multilayered foamsheet. The latter may then be aged for a period of time prior to furtherprocessing, if any.

The process and materials utilized to form the foaming mixture are thesame as those described earlier herein in the process for producing thesingle layer foam sheet. The polypropylene foam layers may be formed byuse of tandem extruders, as in the single layer process. The functionallayers utilized in the process for producing multilayered polypropylenefoam sheets are preferably plasticated in separate, additionalextruders. In addition, tie layers utilized between functional andpolypropylene foam layers also may be fed from separate extruders.

In accordance with the invention, the polypropylene foaming mixture andthe functional resin are combined by means of a multimanifold die, whichhas multiple inlet ports, or a combining feedblock, each of which iswell known in the art. In addition, if the use of tie layers is desired,these materials may also be fed to the multimanifold die or combiningfeedblock. By means of the die or feedblock, three or more layerstructures can be co-extruded by utilizing either an annular die withmultiple inlet ports or a flat die utilizing multiple inlet ports or acombining feedblock. By the use of such equipment, it is possible toproduce a multilayered foam structure having, for example, an outsidepolypropylene foam layer and an inside polypropylene foam layer with afunctional layer sandwiched therebetween. In addition, tie layers may beutilized between the functional and polypropylene foam layers.

A skin may be formed on an outside polypropylene foam layer which ispart of a multilayer structure in a similar manner to that used with asingle layer structure.

The foaming mixture for the preparation of the polypropylene foam sheetof the present invention consists of the polypropylene resin, blowingagent and nucleating agent.

The molecular and rheological characteristics of the polypropylene usedin the process of the present invention, have been describedhereinbefore and include at least

(a) either M_(z) ≧1.0×10⁶

or M_(z) /M_(w) ≧3.0

and

either J_(eo) ≧12×10⁻⁵ cm² /dyne

or Sr/S≧5×10⁻⁵ cm² /dyne at 1 sec⁻¹

In addition, operable polypropylenes include those having a bimodalmolecular weight distribution, wherein the higher molecular weightfraction contains branched polymer.

Although the physical form of the resin is not significant, pelletizedresin is preferred. The melt flow rate of the resin ranges from 0.2 to12.0 g/10 min (2.16 kg, 230° C.).

The nucleating agent, which creates sites for bubble initiation,influences the cell size of the foamed sheet. The nucleating agentsinclude a mixture of citric acid and sodium bicarbonate, talc andtitanium dioxide. Other inert solids used in the prior art and citedherein may also be used in the process of the present invention. Thecited prior art nucleating agents are incorporated herein by reference.It is preferred that the nucleating agent have a particle size in therange of 0.3 to 5.0 microns and that its concentration be less than 1byweight. This plays a role in the formation of 5-18 mil cells, suitablefor thermoforming into rigid or semi-rigid articles for food serviceapplications. Higher concentrations of nucleating agents result in afiner cell structure and the possibility of agent aggregation. When talcand other inert solids are used as nucleating agents, the concentrationis somewhat higher than when the citric acid-sodium bicarbonate mixtureis used.

The blowing agent used in the foaming mixture is a physical blowingagent which is either a gas or undergoes a phase change from liquid togas during the foaming process. The blowing agent is used primarily forcontrolling the density of the foam. The agent dissolved in the polymerunder high pressure and temperature comes out of solution creatingbubbles when the pressure and temperature decrease. The physical blowingagent acts as a plasticizer reducing the viscosity and lowering thetemperature necessary to maintain the hot melt or plasticated conditionof the foaming mixture.

The blowing agents used in accordance with the present invention includehydrocarbons such as butane and isopentane, chlorinated hydrocarbons,chlorofluorocarbons, nitrogen, carbon dioxide and other inert gases. Theconcentration of blowing agent is between 1 and 25% by weight,preferably 2-15%. The higher the concentration of the blowing agent, thelower the foam density due to the combined effects of higher pressure inthe cell and the lower resistance of the cell wall to deformationbecause of the plasticizing action of the blowing agent. An increase inblowing agent concentration in the melt reduces the melt viscosity andthe processing temperature.

The crystallization temperature from the melt is the lower limit inprocessing temperature. When crystallization occurs in the melt, thefoaming characteristics change drastically, usually resulting in surfaceroughness and non-uniform cell size. A conventional non-nucleatedpolypropylene melt starts crystallizing at around 120° C. with a peak ataround 110° C., while the polypropylene resins which are effective inthe preparation of the foam sheets of the present invention, start tocrystallize at about 140° C. with a peak at around 130° C. The lower themelt temperature in the foaming process, the finer the cell size and thelower the open cell content. The lower limit for the polypropylene,isopentane, citric acid-sodium bicarbonate system is about 138° C.

The extruded polypropylene foam sheets of the present invention are agedfor a period of time, e.g. 72 hours, before sheet characterization andthermoforming.

The density of the polypropylene foam sheet of the present invention is2.5-25.0 lbs/ft³ (ASTM D1622) and the thickness is 0.020-0.200 inches(ASTM D645). The tensile and flexural moduli of the foam sheet are inthe 10,000 to 50,000 psi range.

The average size of the cells in the polypropylene foam sheet isdetermined by scanning electron microscopy. SEM micrographs are taken ofcross sections of the sheet, examining both the machine and crossmachine directions. The cell size is determined by counting the cells ina specific area, calculating the average area per cell and convertingthe area per cell into an average cell diameter. The cell diameters forthe machine and cross machine directions are then averaged.

The average size of the cells in the polypropylene foam sheets of thepresent invention ranges from 5 to 18 mils and the microcellularstructure is quite uniform. The open cell content, i.e. volume fractionof ruptured cells in the foam sheet, as determined using an airpycnometer, is about 40% or less.

Acceptable foams, prepared from polypropylene resins having the specificmolecular and rheological characteristics described hereinbefore, areshown in FIG. 2 and 3. Unacceptable foams, prepared from polypropyleneresins not having the requisite characteristics, are shown in FIG. 4 and5.

In accordance with the present invention, the polypropylene foam sheetof the invention can be utilized to form rigid or semi-rigid articles.Such articles can be formed by a thermoforming process which comprisesheating the foam sheet of the invention to a temperature where it isdeformable under pressure or vacuum, supplying the softened foam sheetto a forming mold and cooling the foamed sheet to form a rigid orsemi-rigid article having the shape of the forming mold. The excess foamsheet material, if any can then be trimmed from the shaped article.

The following illustrative embodiments are given for the purpose ofillustrating the instant invention and the invention is not limited toany of the specific materials or conditions used in the examples.

The various polypropylene resins employed in the illustrative andcomparative examples are listed in Table 1, together with theirmolecular and rheological characteristics.

                                      TABLE 1                                     __________________________________________________________________________    CHARACTERIZATION OF POLYPROPYLENE RESINS                                      Resin    A-1 A-2 A-3    A-6    A-7    A-10   A-17   A-20                      Manufacturer                                                                           Himont                                                                            Himont                                                                            Himont HMS                                                                           Himont HMS                                                                           Himont HMS                                                                           Himont HMS                                                                           Exxon HMS                                                                            Himont                    Designation                                                                            6431                                                                              6331                                                                              XA 17054                                                                             X8603-78-1                                                                           X8603-74-1                                                                           17106-2                                                                              PD020  6523                      __________________________________________________________________________    MFR, g/10 min                                                                          7.5 12.0                                                                              9.0    3.4    7.0    7.0    0.3    4.0                       (230° C., 2.16 kg)                                                     [η], dl/g                                                                          1.26                                                                              1.10                                                                              1.18   1.04   1.07   1.01   1.77   1.61                      M.sub.w (×10.sup.3)                                                              241.0                                                                             219.4                                                                             419.6  365.8  326.2  306.1  382.3  315.5                     M.sub.n (×10.sup.3)                                                              39.1                                                                              41.0                                                                              41.2   41.3   41.1   42.7   48.4   41.1                      M.sub.w /M.sub.n                                                                       6.17                                                                              5.36                                                                              10.20  8.86   7.92   7.15   7.90   7.68                      M.sub.z (×10.sup.3)                                                              665.8                                                                             546.7                                                                             1556.9 1455.6 1257.9 1214.1 980.5  852.0                     M.sub.z /M.sub.w                                                                       2.76                                                                              2.49                                                                              3.71   3.98   3.87   3.97   2.57   2.70                      J.sub.eo (×10.sup.-5),                                                           8.8 1.0 19.0   22.0   14.0   15.0   3.3    3.7                       cm.sup.2 /dyne                                                                Sr/S (×10.sup.-5) at                                                             4.0 3.2 8.8    10.5   5.5    8.1    2.9    2.8                       1 sec.sup.-1, cm.sup.2 /dyne                                                  GPC      monomodal                                                                             bimodal                     monomodal                        Structure                                                                              linear  major - linear; minor - branched                                                                          linear                           __________________________________________________________________________

EXAMPLE 1

"High melt strength" (HMS) polypropylene resin, identified as A-6 inTable 1, having an MFR of 3.4 g/10 min, was processed into foam sheets,in accordance with the following procedure.

Resin pellets were fed at the rate of 115 lbs/hr from a resin feederinto a 21/2 inch primary extruder with a 32/1 L/D screw. Astoichiometric mixture of citric acid and sodium bicarbonate was addedsimultaneously at the rate of 0.18 lbs/hr from a separate feeder intothe same port in the extruder. The heating zones in the primary extruderwere maintained at 350° F. (zone 1), 370° (zones 2) and 410° F. (zones3-5). The screw speed was 135 rpm and the internal pressure in theextruder was 2600 psi. Isopentane was injected into the center zone ofthe primary extruder at the rate of 4.4 lbs/hr. After plastication andmixing in the primary extruder, the mixture to be foamed was transferredat a melt temperature of 372° F. through the crossover which was at 355°F. into a 31/2 inch secondary extruder with a 24/1 L/D screw. The fourheating zones in the secondary extruder were maintained at 345° F., 340°F., 330° F. and 325° F., respectively. The screw speed was 19 rpm andthe pressure in the extruder was 800 psi. The foaming mixture at a melttemperature of 283° F. was pumped into the die which was maintained at325° F. The speed of the melt pump was 50 rpm and the die pressure was200 psi.

The mixture was extruded through a circular die with a die lip diameterof 3.0 inches. The foamed tubular extrudate, which was cooled by 70° F.air from an air ring attached to the die lip, was pulled over an 8 inchdiameter cooling/sizing drum which was maintained at about 73° F. Thedistance between the die and the drum was 6 inches. The outer surface ofthe extrudate was cooled by 70° F. air from an air ring around the drum.The tubular extrudate was taken up on a winder after going through anS-wrap. Alternatively, the tubular extrudate was slit as it passed overthe drum and was then taken up on a winder. The take-up speed was 8.9feet/min. The time for passage of the resin through the primary andsecondary extruders was about 20 min.

The polypropylene foam sheet, prepared as described above, had a smoothsurface and a uniform cell size and distribution, as shown in the SEMmicrograph in FIG. 2. The properties of the foam sheet, determined aftera 72 hr aging period, are summarized in Table 2 in Example 5.

EXAMPLE 2

HMS polypropylene resin, identified as A-7 in Table 1, was processedinto foam sheet in the same manner as described in Example 1.

The resin pellets were fed into the primary extruder at a rate of 117lbs/hr while the citric acid-sodium bicarbonate mixture was fedseparately at the rate of 0.61 lbs/hr. Zones 1-5 in the extruder weremaintained at 340° F., 370° F., 410° F., 420° F. and 420° F.,respectively. The screw speed was 140 rpm and the extruder pressure was1730 psi. Isopentane was injected at the rate of 2.70 bls/hr. Themixture to be foamed was transferred at a melt temperature of 288° F.through the crossover which was at 350° F. into the secondary extruder.The four zones in the extruder were maintained at 320° F., 328° F., 325°F. and 325° F., respectively. The screw speed was 13 rpm and theextruder pressure was 830 psi. The molten foaming mixture at 288° F. waspumped into the die which was at 325° F. The die pressure was 190 psiand the melt pump speed was 50 rpm. The mixture was extruded through thecircular die and pulled over the cooling/sizing drum which wasmaintained at about 78° F. The tubular extruder was taken up at a linespeed of 9.3 ft/min. The foam sheet had a smooth surface and a uniformcell size and microstructure, as shown in the SEM micrograph in FIG. 3.The properties of the foam sheet are summarized in Table 2 in Example 5.

EXAMPLE 3--Comparative

HMS polypropylene resin, identified as A-17 in Table 1, was processedunder the same conditions as described in Example 1. The resultantfoamed extrudate could not be pulled over the cooling/sizing drum. Thesheet was a very poor foam and had a lumpy surface and a non-uniformmicrostructure, as shown in the SEM micrograph in FIG. 5.

EXAMPLE 4--Comparative

A conventional polypropylene resin, identified as A-20 in Table 1, wasprocessed under the same conditions as described in Example 1. Theresultant extrudate could not be pulled over the cooling/sizing drum.The sheet was a very poor foam and not acceptable for thermoforming orother applications because of its poor appearance, rough surface andnon-uniform microstructure.

EXAMPLE 5

The characteristics of the resins used in Examples 1-4 are shown inTable 2. The properties of the polypropylene foam sheets prepared inillustrative Examples 1 and 2 from resins A-6 and A-7, respectively, aresummarized in Table 2. The polypropylene foam sheets prepared incomparative Examples 3 and 4 from resins A-17 and A-20, respectively,were very poor and unacceptable in appearance and the properties couldnot be determined.

                                      TABLE 2                                     __________________________________________________________________________    POLYPROPYLENE RESIN AND FOAM SHEET PROPERTIES                                            Illustrative                                                                              Comparative                                            Example    1     2     3      4                                               Resin No.  A-6   A-7   A-17   A-20                                            __________________________________________________________________________    Resin properties                                                              M.sub.z (×10.sup.3)                                                                1445.6                                                                              1257.9                                                                              980.5  852.0                                           M.sub.z /M.sub.w                                                                          3.98  3.87  2.57   2.70                                           J.sub.eo (×10.sup.-5),                                                             22.0  14.0  3.3    3.7                                             cm.sup.2 /dyne                                                                Sr/S (×10.sup.-5) at                                                               10.5  5.5   2.9    2.8                                             1 sec.sup.-1, cm.sup.2 /dyne                                                  Foam sheet properties                                                                    acceptable                                                                          acceptable                                                                          unacceptable                                                                         unacceptable                                    Surface    smooth                                                                              smooth                                                                              rough  rough                                           Microstructure                                                                           uniform                                                                             uniform                                                                             non-uniform                                                                          non-uniform                                     Thickness, mils                                                                            115    83                                                        Denisty, lb/ft.sup.3                                                                     11.2  14.8                                                         Flexural modulus, psi                                                                    18,715                                                                              29,400                                                       Cell size, mils                                                                          15.1   9.6                                                         Open cell content, %                                                                     15.8  22.0                                                         __________________________________________________________________________

The data clearly show that acceptable foam sheets were produced inExamples 1 and 2, wherein the polypropylene resins had the requisitemolecular and rheological characteristics disclosed hereinbefore, whilepoor, unacceptable foam sheets were produced in Examples 3 and 4,wherein the resins possessed none or only one of these characteristics.Although resin A-17, used in Example 3, is a high molecular weightpolypropylene, marketed as a "high melt strength" resin and has an M_(z)value very close to the lower limit required for the preparation ofacceptable foams, the other characteristics, particularly therheological properties, are far below the necessary levels and theresultant foam sheet was very poor and unacceptable, as shown in FIG. 5.

EXAMPLE 6

HMS polypropylene resin, identified as A-6 in Table 1, was processed inthe same manner as described in Example 1. The resin pellets werecharged into the extruder at a rate of 118 lbs/hr while the citricacid-sodium bicarbonate nucleating agent mixture was added at the rateof 0.22 lbs/hr. The blowing agent isopentane was injected at the rate of2.3 lbs/hr. The die pressure was 190 psi and the temperature of the meltpumped into the die was 284° F. The extrudate was pulled over thecooling/sizing drum and taken up on the winder at a speed of 8.2 ft/min.

The polypropylene foam sheet had a smooth surface and a uniform cellsize and microstructure. The thickness of the foam sheet was 113 mils,the density was 12.3 lbs/ft³ and the flexural modulus was 24,485 psi.The average cell size was 10.4 mils and the open cell content was 25.3%.

EXAMPLE 7--Comparative

A conventional polypropylene resin, identified as A-2 in Table 1, wasprocessed in the same manner as described in Example 1. The resinpellets were charged at the rate of 102 lbs/hr while the citricacid-sodium bicarbonate nucleating agent and the isopentane blowingagent were charged at the rate of 0.83 and 2.70 lbs/hr, respectively.The extrudate had a very poor appearance with a rough surface and anon-uniform cell structure, as shown in the SEM micrograph in FIG. 4.

EXAMPLE 8

HMS polypropylene resin identified as A-10 in Table 1, having an MFR of7.0 g/10 min, was processed in the same manner as described inExample 1. The resin pellets, citric acid-sodium bicarbonate andisopentane were charged at the rate of 117, 0.30 and 2.9 lbs/hr,respectively. Zones 1-5 in the primary extruder were maintained at 350°F., 380° F., 410° F., 410° F. and 410° F., respectively, the screw speedwas 153 rpm and the pressure in the extruder was 1750 psi. The mixtureto be foamed was transferred at a melt temperature of 360° F. throughthe crossover, which was maintained at 350° F., into the secondaryextruder. Zones 1-4 in the latter were maintained at 320° F. The screwspeed was 20 rpm and the pressure in the extruder was 800 psi. Themixture at a melt temperature of 285° F. was pumped into the die whichwas maintained at a temperature of 320° F. The speed of the melt pumpwas 50 rpm and the die pressure was 220 psi. The extrudate from acircular die was cooled by 70° F. air as it exited the die and as it waspulled over the cooling/sizing drum. The latter was maintained at 80° F.The take-up speed was 10.0 ft/min.

The polypropylene foam sheet had a smooth surface and a homogeneous cellstructure. The sheet thickness was 101 mils, the density was 8.55 lbs/ftand the flexural modulus was 13,700 psi. The average cell size was 12.4mils and the open cell content was about 40%.

EXAMPLE 9--Comparative

Conventional polypropylene resin identified as A-1 in Table 1, wasprocessed in the same manner as described in Example 1. The foamedextrudate had an uneven surface and a non-uniform cell structure.

EXAMPLE 10

HMS polypropylene resin identified as A-3 in Table 1, having an MFR of9.0 g/10 min, was processed into foam sheets using the tandem extrusionline and conditions described in Example 1, with chlorodifluoromethane(HCFC22) as blowing agent and talc as nucleating agent. The foamingmixture contained 110 lbs resin, 7.10 lbs HCFC22 and 2.90 lbs talc. Thefoam sheet prepared from resin A-3 had a smooth surface and the cellstructure was homogeneous. The sheet had a thickness of 96 mils, adensity of 6.3 lbs/ft³ and a flexural modulus of 12,700 psi.

EXAMPLE 11--Comparative

HMS polypropylene resin identified as A-17 in Table 1, having an MFR of0.3 g/10 min but lacking the requisite rheological characteristics, wasprocessed in the same manner as described in Example 10. The resin wascharged at the rate of 85.9 lbs/hr while HCFC22 and talc were charged atthe rate of 6.70 and 3.40 lbs/hr, respectively. The extrudate had a verypoor surface appearance and a non-uniform cell structure. These resultsare similar to those obtained with the same resin using isopentane asblowing agent and citric acid-sodium bicarbonate as nucleating agent, asdescribed in Example 3.

EXAMPLE 12--Thermoforming

Foam sheet having a poor appearance with a rough surface and anon-uniform cell structure could not be thermoformed into shapedarticles with a smooth, acceptable surface. Acceptable, good qualityfoam sheet with a smooth surface and uniform cell structure could bethermoformed into good quality shaped articles.

The good quality polypropylene foam sheet prepared from resin A-6 inExample 1 and having a thickness of 115 mils, a density of 11.2 lbs/ft³,an average cell size of 15.6 mils and an open cell content of 15.8%, wasthermoformed in a Comet Model 24×24 apparatus with matched mold toolingto form an 8 oz round bowl. The foam sheet was placed between the topand bottom sections of the mold which were at a temperature of 290° F.The heating time was 120 sec with a dwell time of 15 sec. The reducedpressure on the sheet in the mold was 20 inches Hg. The thermoformedbowl had a smooth surface and good appearance. Test specimens taken fromthe bottom of the bowl had a tensile modulus of 14,300 psi, a tensilestress of 441 psi, a tensile strain of 4.1% and an energy-to break of19.3 inch-lbs.

EXAMPLE 13

EPO Pat. Application EP 71,981 discloses the preparation of foamedmolded articles from polypropylene resins having a latent heat ofcrystallization of 9-28 cal/g.

Several polypropylene resins identified in Table 1, were subjected todifferential scanning calorimetric analyses to determine the latent heatof crystallization. The results are summarized in Table 3.

                  TABLE 3                                                         ______________________________________                                        LATENT HEAT OF CRYSTALLIZATION OF RESINS                                      Polypropylene  Latent H.sub.cr                                                                         Foam Sheet                                           Resin          cal/g     Quality                                              ______________________________________                                        A-1            23.97     Unacceptable                                          A-17          22.76     Unacceptable                                         A-6            22.62     Acceptable                                           A-7            23.24     Acceptable                                           ______________________________________                                    

It is apparent that the latent heat of crystallization does notdistinguish between those polypropylene resins which yield theacceptable foam sheets of the present invention and those that yieldunacceptable non-uniform foam sheets.

EXAMPLE 14

U.S Pat. 3,637,458 discloses the preparation of a foam sheet of alinear, thermoplastic polymer of film-forming molecular weight,substantially free from crosslinking and having a work-to-break value ofat least 10,000 inch-lbs/in³, and by the process of the cited patent,yielding a foam sheet having a maximum density of 0.03 g/cc (1.87lbs/ft³) and a cell size of at least 500 microns (19.5 mils).

The polypropylene foam sheet of the present invention has a minimumdensity of 2.5 lbs/ft³ and a maximum cell size of 18 mils, both outsideof the claims of the above cited U.S. patent.

Several polypropylene resins, identified in Table 1, were extruded at amelt temperature of 205-250° C., on a twin-screw Brabender extruderrotating at 25-30 rpm, using a ribbon die and chill roll, to producesamples having a thickness of 3-5 mils. The test specimens, 5-9 persample, were conditioned for 1-3 days at 72° F. at 50RH prior to runningInstron tests at a crosshead speed of 0.5 inch/min using a Type IV die(ASTM D638).

As shown in Table 4, those polypropylene resins which yield acceptablefoam sheet when prepared by the process of the present invention, have awork-to-break value of less than 10,000 in-lbs/in³, whereas resinshaving the work-to-break value of at least 10,000 in-lbs/in³ claimed inthe cited patent, yield non-uniform, unacceptable foam sheet.

                  TABLE 4                                                         ______________________________________                                        WORK-TO-BREAK OF POLYPROPYLENE RESINS                                         polypropylene Work-to-break                                                                             Foam Sheet                                          Resin         in-lbs/in.sup.3                                                                           Quality                                             ______________________________________                                        A-1           16,490      Unacceptable                                         A-17         16,010      Unacceptable                                        A-6            6,300      Acceptable                                          A-7            7,446      Acceptable                                          ______________________________________                                    

EXAMPLE 15

U.K. Pat. Application GB 2 099 434A discloses an extrusion process forpreparing a foam extrudate from a polypropylene resin having a melttension of at least 3 grams at 190° C. and a maximum/minimum melttension ratio of not more than 2.5. The melt tension was measured byextruding the molten polymer, heated to 190° C., through a 2 mm orificeand then passing the extrudate through a tension-detecting pulley andwinding it up at a speed of 5 cm/sec.

Two polypropylene resins (A-1 and A-17), which gave unacceptable foamsheets, and two polypropylene resins (A-6 and A-7), which gaveacceptable foams, were subjected to the procedure for measurement of themelt tension at 190° C., as described in the cited U.K. patentapplication. At the temperature and throughput rate specified in thecited application, all of the polypropylene extrudates froze beforereaching the take-up device.

Melt tension values for all four resins could be obtained at a highertemperature. The melt tension results shown in Table 5 were obtained ata melt temperature of 220° C. and a take-up speed of 3 cm/sec.

                  TABLE 5                                                         ______________________________________                                        MELT TENSION AND MOLECULAR WEIGHT                                             PARAMETERS OF POLYPROPYLENE RESINS                                            Polypropylene                                                                 Foam Sheet   Melt       MT.sub.max                                                                            M.sub.n                                                                             M.sub.w                                 Resin Quality    Tension, g MT.sub.min                                                                          (×10.sup.3)                                                                   (×10.sup.3)                     ______________________________________                                        A-1   Unacceptable                                                                             1.5        1.33  39.1  241.0                                 A-7   Acceptable 4.1        1.17  41.1  326.2                                 A-6   Acceptable 8.5        1.13  41.3  365.8                                  A-17 Unacceptable                                                                             15.0       1.06  48.4  382.3                                 ______________________________________                                    

Although the absolute melt tension values would not be expected to bethe same at 220° C. and at 190° C., the relative ranking should be thesame. The data show that the melt tension values and the maximum/minimumratio do not distinguish among those polypropylene resins which yieldacceptable foams and those that yield unacceptable foams.

Molecular weight parameters for the various resins are shown in Table 5.It is apparent that the melt tension values increase with increasingmolecular weight, as measured by M_(n) and M_(w). It has already beenshown hereinbefore that both molecular and rheological characteristicsat specified minimum levels are necessary to define the polypropyleneresins which yield acceptable foams.

Although the density of the foamed product is not discussed in the citedU.K. patent application, the five illustrative examples disclosedensities in the range of 0.023 to 0.036 g/cm³ (1.44 to 2.25 lbs/ft³).The present invention discloses densities of 2.5-25.0 lbs/ft³.

The inability to obtain melt tension values at 190° C. for thepolypropylene resins used herein, while such values are reported in theU.K. patent application, suggests that the resins reported therein werenot polypropylene but were "polypropylene-type resins".

The polypropylene used in the examples in the cited patent applicationare not identified as to source, composition, etc. The polypropyleneresins disclosed as usable in the process of the cited application areresins composed mainly of polypropylene including isotacticpolypropylene, ethylene-propylene block copolymer, ethylene-propylenerandom copolymer and mixtures of two or more of the above mentionedpolypropylene-type resins. In addition, polymers miscible with thesepolypropylene-type resins, such as high and low density polyethylenes,polybutene-1, ethylene copolymers with vinyl acetate, propylene andethyl acrylate, styrene-butadiene rubber, ionomer and the like may bemixed with the above mentioned polypropylene-type resins, so far aspolypropylene is the main component in the resulting mixture.

The polypropylene used in the cited U.K. Pat. Application GB 2 099 434Amay have been one or more of the copolymers or mixtures disclosedtherein and above.

What is claimed is:
 1. A foam sheet of a polypropylene characterized byat least(a) either z-average molecular weight M_(z) of at least 1.0×10⁶or a ratio of the z-average molecular weight M_(z) to weight averagemolecular weight M_(w) M_(z) /M_(w) of at least 3.0, and (b) eitherequilibrium compliance J_(eo) of at least 12×10⁻⁵ cm² /dyne orrecoverable shear strain per unit stress Sr/S of at least 5×10⁻⁵ cm²/dyne at 1 sec⁻¹.
 2. A foam sheet of polypropylene having a biomodalmolecular weight distribution, wherein the higher molecular weightfraction contains branched polymer.
 3. The foam sheet of claim 1,wherein said sheet has a density of 2.5-25.0 lbs/ft³ and a flexuralmodulus of 10,000-50,000 psi.
 4. The foam sheet of claim 1, wherein saidsheet has a uniform cellular structure with cells ranging from 5 to 18mils and an open cell content of less than 40%.
 5. The foam sheet ofclaim 1, wherein said sheet has a thickness of 0.020-0.200 inches.